There are a variety of conventional bonding techniques for bonding semiconductor material or metal surface to substrate. One of them is the anisotropic conductive film (ACF) bonding technique. The ACF bonding technique places a layer of anisotropic conductive film containing conductive particles between chip and the device to be bonded, and bonds the chip and the device together by melting the anisotropic conductive film using heat and pressure. It also forms conductive channel by using metal pads, metal bumps and conductive particles. The disadvantage of this technique is that it can not meet finer pitch requirement. For a finer pitch between the metal pads and the metal bumps, conductive particles will flow because of heat and pressure being applied. Thereby two adjacent conductive points may be short. Thus, the technique can not meet finer pitch requirement. The bonding density of this ACF bonding technique can only reach to as small as 50 μm pitch.
Another conventional bonding technique is using non-conductive film (NCF) for bonding. The difference between the NCF bonding technique and the ACF bonding technique is that the former does not contain any conductive particle in the adhesive material. The bonding structure using this NCF bonding technique uses heat and pressure to melt the non-conductive film. After the non-conductive film has consolidated, the generated contractive stress bonds the chip and the device together. Although the bonding density is high for this NCF bonding technique, the bonding of the chip and the device is maintained only by mechanical force. That is, the contractive stress generated by the film has to maintain the conducting quality of the contact points. Once the film bears too much stress, the contact surface among the film, integrated circuit and substrate will produce lamination and increase the resistance after bonding.
Another method is Au—Au diffusion bonding technique. Because its bonding temperature is too high and metal oxides will be formed on the surfaces of metal layers, covalent bonds will limit free electrons of metal. Therefore, it is hard to form metal bonds between two bonding surfaces. Also, the electrical conductivity comes from tunnel-through effect that generates higher contact impedance. Therefore, it is not suitable to fine pitch applications too.
U.S. Pat. Nos. 5,407,506 and 5,427,638 disclose respectively surface activation methods. The surface activation methods disclosed in these U.S. patents mainly bombard the polished surfaces and the cleaned surfaces by oxygen ions, fluorine ions, or their mixture to activate these surfaces. Then particles on these activated surfaces are removed and these activated surfaces are contacted under room temperature to complete the activation bonding.